On the Communication of the Structure of doubly refracting Crystals to Glass, Muriate of Soda, Fluor Spar, and other Substances, by mechanical Compression and Dilatation. By David Brewster, LL.D. F.R.S. &c. In the introduction to this paper, Dr. Brewster remarks that, notwithstanding the numerous discoveries which have been made relative to the polarization of light, and the optical phænomena of crystallized bodies, not a single step has hitherto been gained towards the solution of the great problem of double refraction. What is the mechanical condition of crystals that form two images and polarize them in different planes? and what are the mechanical changes which must be induced on uncrystallized bodies, in order to communicate to them these remarkable properties? are questions which have remained till the present time unanswered: but Dr. Brewster thinks that he is now enabled to solve them in the most satisfactory manner; as well as to communicate to glass, and many other substances, by the mere pressure of the hand, all the properties of the different classes of doubly refracting crystals.

Some of the most interesting of these experiments are contained in the heads of the first, second, and third propositions:

I. If the edges of a plate of glass, which has no action upon polarised light, are pressed together or dilated by any kind of force, it will exhibit distinct neutral and depolarising axes like all doubly refracting crystals, and will separate polarised light into its complementary colours. The neutral axes are parallel and perpendicular to the direction in which the force is applied, and the depolarising axes are inclined to these at angles of 45°.

II. When a plate of glass is under the influence of a compressing force, its structure is the same as that of one class of doubly refracting crystals, including calcareous spar, beryl, &c.; but when it is under the influence of a dilating force, its structure is the same as that of the other class of doubly refracting crystals, including sulphate of lime, quartz, &c.'

III. If a long plate or slip of glass is bent by the force of the hand, it exhibits, at the same time, the two opposite structures described in the preceding proposition. The convex or dilated side of the plate affords one set of coloured fringes, similar to those produced by one class of doubly refracting crystals; and the concave or compressed side exhibits another set of fringes, similar to those produced by the other class. These two sets of fringes are separated by a deep black line where there is neither compression nor dilatation.'

To this latter proposition, the author has added a scholium, endeavouring to shew that the tints produced by polarized light are correct measures of the compressing and dilating forces; and that the theory of the strength of materials and REV. DEC. 1816.

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the cohesion of solids will be farther illustrated by the principles previously established.

There is one practical application of these views which is particularly deserving of notice. In order to observe the manner in which stone arches yield to a superincumbent pressure, Dr. Robison executed several models in chalk, and deduced many general laws relative to the internal forces by which they were crushed. If the arch stones of models are made of glass, or any other simply refracting substance, such as gum copal, &c. the intensity and direction of all the forces which are excited by a superincumbent load in different parts of the arch will be rendered visible by exposing the model to polarised light. If different degrees of roughness are given to the touching surfaces of the glass voussoirs, the results may be obsérved for any degree of friction at the joints. The intensity and direction of the compressing and dilating forces, which are excited in loaded framings of carpentry, may be rendered visible in a similar manner.'

After the terms in which we have spoken of Dr. Brewster's experiments and discoveries, we shall not be suspected of wishing to undervalue either his talents or the utility of his researches and deductions, when we say that we must consider this notion as a little fanciful; and that we should very much doubt the probability of ever obtaining any useful information on these subjects, if it were to be acquired only through the medium of polarized light.

Of the subsequent propositions, the heads are thus stated:

IV. The tints polarised by plates of glass, in a state of compression or dilatation, ascend in Newton's scale of colours as the forces are increased; and in the same plate, the tint polarised at any particular part is proportional to the compression or dilatation to which that part is exposed.'

V. When compressed and dilated plates of glass are combined transversely and symmetrically, they exhibit all the phenomena which are produced by the combination of plates of doubly refracting crystals.'

VI. If a plate of glass resting on two supports is bent by any force applied between the points of support, the tints are a maximum at the part where the pressure is applied, and ascend gradually in the scale of colours towards the points of support.'

VII. If a plate of glass is subject to compressions or dilatations exerted in different directions, the same effects are produced as when separate plates influenced by the same forces are combined in a similar manner.'

VIII. If two plates of bent glass are placed together at their coneave or compressed edges, the compound plate has exactly the same properties as a plate of glass transiently or permanently crystallized by heat, which gives the usual series of fringes. But if the two plates are placed together at their convex or dilated edges, the compound

plate

plate has the same properties as plates of glass transiently crystal lized by heat, which produce the unusual series of fringes.'

IX. If the compressing and dilating forces are applied to the centre of a plate of glass, the principal axes of the particles will be directed to the point of compression or dilatation, and the glass will exhibit the black cross, and the other phenomena which are seen in doubly refracting crystals.'

X. If a plate of glass in a state of compression or dilatation is inclined to the polarised ray in a plane parallel to the axis of dilatation and compression, the tints will descend in the scale; but if they are inclined in a plane at right angles to these axes, the tints will ascend.'

XI. If a plate of glass that has already received the doubly refracting structure from heat, is exposed to compression, the tints of the interior fringes rise in the scale, and those of the exterior fringes descend, when the axis of pressure is perpendicular to the direction of the fringes; the opposite effect being produced by a dilating force. The same results are in this case obtained as if an uncrystallized plate, similarly compressed or dilated, had been similarly combined with the crystallized plate.'

'XV. If a parallelopiped of glass is enclosed on all sides, except two, in a mass of fluid metal, the contractions and dilatations which the metal experiences in passing to a state of permanent solidity, will be rendered visible by the communication of the doubly refracting structure to the glass.'

Of the part of the present memoir that explains the principles of certain new instruments, which the author calls the chromatic dynamometer, chromatic thermometer, and chromatic hygrometer, we cannot attempt farther explanation; and we must also pass various propositions relating to the communication of double refraction, both transiently and permanently, to animal jellies by gradual induration, and by mechanical compression and dilatation.'

The limits, within which we are under the necessity of confining our abstracts and remarks, will not enable us to give a more connected view of Dr. Brewster's ingenious memoirs, which оссиру above ico pages, filled with new and interesting experiments and deductions: but, in order to shew the statę to which the question relative to double refraction is now reduced, we shall copy the author's concluding observations.

Upon reviewing the general principles contained in the preceding propositions, I cannot but allow myself to hope that they will be considered as affording a direct solution of the most important part of the problem of double refraction. The mechanical. condition of both classes of doubly refracting crystals, and the method of communicating to uncrystallized bodies the optical properties of either class, have been distinctly ascertained, and the only phenomenon which remains unaccounted for is the division of the incident light into two oppositely polarised pencils. How

far

far this part of the subject will come within the pale of experiméntal inquiry, I do not presume to determine; but without wishing to damp that ardour of research which has been so happily directed towards this branch of optics, I fear that, as in the case of electrical and magnetical polarity, we must remain satisfied with referring the polarisation of the two pencils to the operation of some peculiar fluid. The new property of radiant heat which enables it to communicate double refraction to a distant part of a plate of glass, where the heat does not reside in a sensible state; the existence of a moveable polarity in glass, whether the doubly refracting structure is communicated transiently or permanently;and the appearance of regular cleavages varying with the direction of the axes of double refraction, are facts which render it more than probable that a peculiar fluid is the principal agent in producing all the phenomena of crystallization and double refraction.

There is one fact, however, which forms a fine connection between the aberration of the extraordinary ray and the principles established in this paper. It has been demonstrated by an eminent English philosopher, that every undulation must assume a spheroidal form when propagated through a minutely stratified substance, in which the density is greater in one direction than another, and I have proved by experiment that such a substance actually possesses the property of double refraction. This singular coincidence will no doubt be regarded as an argument in favour of the undulatory system.'

The Second Part of these Transactions for the present year has recently made its appearance, and we mean speedily to report its contents.

ART. VII.

An Historical View of the State of the Protestant Dissenters in England, and of the Progress of Free Enquiry and Religious Liberty, from the Revolution to the Accession of Queen Anne. By Joshua Toulmin, D.D. 8vo. pp. 628. 128. Boards. Longman and Co.

THE

HE prolix but copious and authentic work of Neal brought down the History of Nonconformity from the first rise of the Puritans to the passing of the Act of Toleration; and many years ago the author of the volume before us, in publishing a new edition of The History of the Puritans," announced his intention of continuing it to the present times: but his design was long suspended by a succession of obstacles and interruptions, the nature of which is stated with much simple pathos in his preface:

It gives the author concern to reflect that eighteen years have elapsed without affording a proof, by the execution of any part of his intended work, that he was sensible either of the deference which he owed to public expectation, and to the desires of warm friends, or of the obligation brought on him by his own engage

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ment. He consoles himself with believing that they who know him will candidly acquit him of any disrespect to the deceased, or of desultory idleness. His attention, he may be permitted to plead, has been for years diverted from this work by a succession of occurrences and engagements which, unforeseen when it was announced, have either accompanied or followed the anguish of mourning, or the depressions of deep sorrow on the death of children who had just reached the promising years of maturity; especially of a daughter, whose removal was so circumstanced as to create, by a lasting mournful sense of it, a long interval of inaptitude for any continued mental exertion. This afflicting event was succeeded, at different distances, not only by other similar trials, but by avocations that arose from the confidence and trust reposed in him by several deceased friends; by derangement of studies produced by removals from one dwelling to another; and above all by the reading and application to prepare an extensive Course of Lectures for young persons, in which his present pastoral connection engaged him more than ten years ago.'

Dr. Toulmin possessed qualities which peculiarly fitted him to be the historian of religious opinion, and the rise or decline of religious parties. His candid and amiable temper preserved him, perhaps, as free from all tendency to misrepresent the opinions of others for the sake of recommending his own, as it is possible that a man should be who is at the same time the zealous advocate of a particular system, The habit of his mind disposed him more, indeed, towards the minute researches of biography, than to the comprehensive views of history but the effect of this inclination will be less felt in the particular species of history which he undertook than in any other, because the interest which religionists of every denomination feel in the annals of their party arises chiefly from the talents and virtues of those by whom their communion has been adorned. The memory of Baxter, Watts, and Doddridge is cherished by multitudes, who are very little able to appreciate the influence which the existence of a body of Dissenters has had on the political history and moral character of their country, or to speculate on those laws of our nature which are exemplified in the fluctuations of religious opinions. It was the author's design, (as he announced in his preface,) had his life been prolonged, to have continued his details in a second volume to the accession of his present Majesty but his death prevented the fulfilment of that object; and we regret to find that he has left no materials from which his projected plan may be completed, indeed scarcely any that can lessen the labour of the person who may take up the history of the Dissenters where he has left it. It was probably the desire to discharge some part of his engage

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